copia de VM a un disco externo

Yes, it could be related to sparse files.

You can compare the apparent size and the actual disk usage with:

ls -lh vm-100-disk-0.qcow2
du -h vm-100-disk-0.qcow2
 
Yes, it could be related to sparse files.

You can compare the apparent size and the actual disk usage with:

ls -lh vm-100-disk-0.qcow2
du -h vm-100-disk-0.qcow2
That being said, and doing some guesswork @Notaria, FAT32 doesn't support sparse files.

Check which filesystem are you using in the destination disk.
 
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