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I am using format command in cmd to format a USB flash drive.

M:\>format /FS:FAT32 /Q
Required parameter missing -

M:\>format M: /FS:FAT32 /Q
Insert new disk for drive M:
and press ENTER when ready...
The type of the file system is FAT32.
QuickFormatting 14999M

Format cannot run because the volume is in use by another
process.  Format may run if this volume is dismounted first.
Would you like to force a dismount on this volume? (Y/N) y
Volume dismounted.  All opened handles to this volume are now invalid.
Initializing the File Allocation Table (FAT)...
Volume label (11 characters, ENTER for none)?
Format complete.
      14,6 GB total disk space.
      14,6 GB are available.

        8 192 bytes in each allocation unit.
    1 917 823 allocation units available on disk.

           32 bits in each FAT entry.

Volume Serial Number is E00B-2739


Is it safe to force a dismount like this, and make the handles invalid?

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If you're erasing all the data on the drive anyway, (this is one of the things a format does, after all) the validity of any file handles open on it is fairly moot. The only thing to be concerned about at this point is if you were backing up the data before the format, and there are still processes working on that. To be safe, close all other programs before retrying and double-check to be sure that any backups you had going have fully completed. If it still gives issue about open file handles, try a reboot.

If that doesn't work, try rebooting in safe mode. If that doesn't work, pull out your toolkit of choice (e.g.: SysInternals, NirSoft, etc.) and start troubleshooting to find out what keeps opening handles on that drive so you can kill it. Or just reformat the whole machine, because at this point there's definitely something extra-special going on.

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