Answer by jaume for 8GB stick thinks it's a 4GB one

I think the USB drive is going haywire, I'd recommend that you stop using it.

Ubuntu reports 4 instead of 8 GB:

$ sudo fdisk -l /dev/sdb
Disk /dev/sdb: 3763 MB, 3763600896 bytes
116 heads, 62 sectors/track, 1022 cylinders, total 7350783 sectors
(...)

And if you look at fdisk's output more closely you will notice that the start and end sectors make no sense at all and don't match the total number of sectors on the Cruzer:

Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   ?  4281232757  1652651905   833193222+  6d  Unknown
/dev/sdb2   ?  1141509631  1685422960   271956665   66  Unknown
/dev/sdb3   ?  1937007983  1937010555        1286+  65  Novell Netware 386
/dev/sdb4      2885681152  2885736393       27621    0  Empty

Something's wrong with the USB flash drive. I don't see how you can get your 8 GB back.