Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

When I connect my USB 2.0 drive to Xubuntu and try to transfer large files, transfer speeds are good at first but drop after a few seconds to 1-2 Mib/s. From what I read, the fast transfer at the beginning is just until the cache is full, then the real USB transfer speed is used.

In Windows, speed is constant at about 25Mib/s, same stick, connected to same port.

This is the output from dmesg when connecting the stick:

[ 5237.580084] usb 1-4: new high speed USB device using ehci_hcd and address 6
[ 5237.714318] scsi4 : usb-storage 1-4:1.0
[ 5238.713909] scsi 4:0:0:0: Direct-Access     SanDisk  Cruzer           8.02 PQ: 0 ANSI: 0 CCS
[ 5238.715264] sd 4:0:0:0: Attached scsi generic sg2 type 0
[ 5238.727225] sd 4:0:0:0: [sdb] Attached SCSI removable disk
[ 5242.308981] sd 4:0:0:0: [sdb] 31301631 512-byte logical blocks: (16.0 GB/14.9 GiB)
[ 5242.309589] sd 4:0:0:0: [sdb] Assuming drive cache: write through
[ 5242.311228] sd 4:0:0:0: [sdb] Assuming drive cache: write through
[ 5242.311238]  sdb: sdb1

The stick is automounted, here is the output of "mount":

/dev/sdb1 on /media/B82C-6B07 type vfat (rw,nosuid,nodev,uhelper=udisks,uid=1000,gid=1000,shortname=mixed,dmask=0077,utf8=1,showexec,flush)

I have to use vfat because I want to transfer files to my Playstation3 system. Althouth the test below shows, that this is not the main cause.

It seems this is a common issue in (X)ubuntu. I haven't found a clear solution yet. It seems that the stick needs to be mounted as async instead of flush but I'm not exactly sure how to achieve this. I don't mind if I have to unmount the stick each time before disconnecting it as long as the transfer speeds are better.

Any ideas?

Nov 2: This bug report seems to be related: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/182931. What worries me is that the problem seems to be over 2 years old and still isn't solved.

Nov 10: I started a read-test with palimpstest (from gnome-disk-utility project). This gives me a minimal transfer rate of 23 MBit/s. Also tried it in krusader now, the read speed is constantly > 20 MBit/s. So the problem really is just writing to the stick!

Here is an image of a read-write test in the same program:

enter image description here

I had to delete the filesystem completely before the test which shows that vfat is not the main cause as mentioned above.

Another hint: If I transfer a 100 MB file, once it's at 100% it takes an unusual long time to actually finish the transfer. This again looks to me that the file is indeed in a cache but writing to the usb device is very slow.

Nov 12: "Good news". The problems seems to be memory stick itself. Trying an external harddrive (My Book Elite 1TB) gives me constant write speed of about 20 MiB/s. I first tried to reformat my memory stick so I can really compare the two (file sytems were different). After reformatting, both drives are mounted exactly the same way:

/dev/sdb1 on /media/My Book type fuseblk (rw,nosuid,nodev,allow_other,blksize=4096,default_permissions)
/dev/sdc1 on /media/cruzer_ type fuseblk (rw,nosuid,nodev,allow_other,blksize=4096,default_permissions)

Cruzer is the stick (speed still dropping after a few seconds), My Book is the external harddrive with a constant write speed of 20 MiB/s.

Hard to say what the problem really is then. As I said, in Windows I get equally fast write speeds with the SanCruzer too.

Edit at end of bounty: Unfotunately, the problem with the stick is still not solved. However, after 7 days I want to award the bounty of course. The comment from aking1012 was really helpful as it shed a new light on the matter. However, I feel that I have to give the bounty to harrymc as he showed the most effort of getting the problem solved. Thanks all.

share|improve this question
ocaoimh.ie/ubuntu-linux-slow-external-usb-drive Hope this helps =) –  user54120 Oct 31 '10 at 20:09
is there a difference between the write speed for flash drives and true external HDs...just a question –  hbdgaf Nov 12 '10 at 17:28
Excellent idea! Thanks a lot for the tipp, please see the result in my update (Nov 12). The problem is not solved yet, but this really narrows it down! –  socken23 Nov 12 '10 at 18:57
FWIW, the flush option in your mount output is unlikely to help. I think that's even worse than sync. –  beldaz Jul 21 '14 at 10:31

2 Answers 2

The article Slow USB 2.0 mass stoarage transfer rate suggests that this might be a problem of the device getting mounted by default with the sync option :

Do you have a "Devices" Icon on your Desktop?
If so, open it. If not, try opening media:/ in konqueror.

Selct the device then -> Properties -> Mounting(tag) and uncheck the Synchronous box. (This gets remembered, so you should only need to do it once per-device).

Some more ideas come from the bug-report Write through on external hdd making writing very slow

  • you change to async at runtime by 'sudo mount -o remount,async /dev/sda1' (or whatever your mounted partition(s) is/are)
  • you create an fstab entry for sda which mounts it async (this requires that the device is always plugged in at boottime)
  • you change 'sync' to 'async' in the pmount source and rebuild the package

See also the discussion in the article as regarding pmount, if this applies to your system.

Another magic solution comes from USB 2.0 slow write but fast read, solution yet? and involves re-mount:

When I mount a USB drive on /dev/sdc1 with the following line

/dev/sdc1       /mnt/sdc1     auto    sync,noauto,user,exec     0       0

in /etc/fstab, and try to copy a 5.2 MB file to the drive from the local hard disk, then after an entire minute, 244 KB have been transferred. Copying a 29 MB file from the external drive to the local drive takes about 10 seconds.

Then if I unmount it, change the line in /etc/fstab to

/dev/sdc1       /mnt/sdc1     auto    noauto,user,exec     0       0

then copying the 29 MB file to the external disk takes an unnoticeable fraction of a second. Same with copying to the local disk from the external disk.

See this article and the reference to "usbtree". It tells how to check if the usb is functioning as 1.1 or 2.0 :

Linux and USB 2.0.

share|improve this answer
It seems this is specific for KDE. I'm using XFCE4 (Xubuntu). I tried the suggestion anyway, as Konquerer is installed. opening media:/ gives me the exception "Protocol not supported". Any other way I can find out if it is mounted "async" or "sync" ? –  socken23 Nov 10 '10 at 9:37
I added one more useful article. –  harrymc Nov 10 '10 at 9:55
remount doesn't do anything. The output of "mount" is still the same. See my initial message for it. I also tried to mount it with pmount now, thanks for the suggestion. The help of pmount states, that it mount async by default. Still, transfer speeds are very fast first and drop after a few seconds. I'm starting to belive that the stick is actually mounted async but the problem is somewhere else!? –  socken23 Nov 10 '10 at 10:33
Added one more try. –  harrymc Nov 10 '10 at 18:22
Thanks harrymc, really appreciate your continous effort. Unfortunately, still not a chance. I tried it with the mentioned settings in /etc/fstab and mounting it. If I try the option with sync, it's even worse. I only get around 100-200 Kib/s. Without the sync option, it's still the same: fast in the beginning, dropping to 1-2 MiB/s. –  socken23 Nov 10 '10 at 19:20

Access to the drive is usually buffered in async mode. The initial fast burst is likely to to memory cache. Slow speed is actual write speed.

If you have a USB stick which flashes during transfer try some tests. I found the writes can continue for quite a while after the copy is completed.

Mounting in async mode instead of sync mode allows the copy to appear to proceed faster, but doesn't give you the same security that copied data is really transferred.

Check the options in man mount. Some formats allow metadata (directory, file allocation table, etc,) updates to be deferred. This will cut down the amount of data that needs to be copied. This increases risk of data loss slightly.

share|improve this answer
Thanks for the info, this actually confirms my assumptions. The drive flashes constantly during the transfer, it doesn't flash long after the transfer though. I suspect this only to be because the file manager is still accessing the stick. Security wouldn't be a big problem, I'm really just interested in the speed. Interesting about the metadata, I will check that. Although, do you think this will change anything when transferring large files (500 MiB) ? –  socken23 Nov 16 '10 at 9:45
Also, what still puzzles me, why is it very fast with the external harddisk but not with the stick? The mount options are exactly the same, I transferred the exact same test file. –  socken23 Nov 16 '10 at 9:50
A 4 KiB write to a hard drive is typically a write of 4 KiB, implemented by the hard disk controller. A 4 KiB write to a Flash memory is typically a 128 KiB read to some temporary storage, a 4 KiB overwrite at correct offset of said temp storage, a 128 KiB erase operation, and a 128 KiB write, all of the above implemented by the Flash memory controller. More or less :) –  tzot Oct 2 '12 at 22:08

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.