I just got a new larger external hard drive and transferred all the files from my old drive to the new one.

When I was checking the used space to make sure everything transferred, I found that the new drive was about 70gb larger than the old one, despite being entirely empty previous to backing up the old drive onto it.

Upon further investigation I found that while every file said it was the same size, the "__ space on disk" did not match the files byte size. Here's an example


Tech specs:

Old Drive: seagate 1 TB HFS+ format 8,03.7 GB used
New Drive: seagate 4 TB exfat format 8,71.0 GB used

I formatted the new one with exfat so it would work with both mac and pc. Is the extra used space caused by the difference in format?

  • The actual (data) size of files very rarely matches the on disk size of files, because of sector block sizes that leave unused space at the end of files. Or sparse files, or some compressed filesystems, etc. What does the same folder's properties look like on the old drive? – Xen2050 Nov 7 '18 at 6:55
  • I also noticed this and as the previous comment mentioned the block size or also know as the the allocation unit size choosen when formatting a hard drive affects the space a file takes up on that hard drive. – Alz Belz Nov 7 '18 at 8:11
  • Why is there such a big difference between “Size” and “Size on disk”?. The question and answers there are mostly Windows-centric, still you can get a good general insight from them. – Kamil Maciorowski Nov 7 '18 at 8:25
  • @Xen2050 the old drive has some differences too but typically much smaller like a couple MB max where as the new drive has differences about a GB or more – zander Nov 7 '18 at 8:59
  • What's the block/sector size of both drives? Filesystem types? – Xen2050 Nov 7 '18 at 9:01

This is likely due to a difference in sector/block size on the disk... Which is probably due to the change in disk format from HFS+ to ExFAT.

Drives can only store a single file in a sector, with larger files spanning many sectors. Files that are smaller than a sector still occupy that sector, which means they use more space on the disk than their actual file size.

| improve this answer | |
  • is there a way to change the sector/block size or is there any way to have the drive reorganize itself so that it uses the space more efficiently? The wasted space is even larger than i thought initially, clocking in at a whopping 130GB of wasted space – zander Nov 7 '18 at 9:42
  • to a degree, yes, some formats allow the changing the default sector size, depending on the OS etc. Don't forget that you'll also be seeing difference from 'reported' sizes between 'bits' and 'bytes'. See this document for a full write up on the issue. lifewire.com/drive-storage-capacities-833435 – Stese Nov 7 '18 at 9:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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