USB sticks have been known to be less reliable than a HDD that is externally powered. I think you’d be better off by using a HDD
I've used them on other routers (including ASUS) without any problems up till now. I don't know whether the following gives any clues, but this is the log from the NTFS formatting (I didn't get one for the FAT formatting because it didn't complete):-
/dev/sda is entire device, not just one partition.
mkntfs forced anyway.
Sector size is 512 bytes.
Volume size is 123174911 sectors (60143 MiB).
Cluster size has been automatically set to 4096 bytes.
Creating NTFS volume structures.
Creating root directory (mft record 5)
Creating $MFT (mft record 0)
Creating $MFTMirr (mft record 1)
Creating $LogFile (mft record 2)
Creating $AttrDef (mft record 4)
Creating $Bitmap (mft record 6)
Creating $Boot (mft record 7)
Creating backup boot sector.
Creating $Volume (mft record 3)
Creating $BadClus (mft record 8)
Creating $Secure (mft record 9)
Creating $UpCase (mft record 0xa)
Creating $Extend (mft record 11)
Creating system file (mft record 0xc)
Creating system file (mft record 0xd)
Creating system file (mft record 0xe)
Creating system file (mft record 0xf)
Creating $Quota (mft record 24)
Creating $ObjId (mft record 25)
Creating $Reparse (mft record 26)
Syncing root directory index record.
Syncing $Bitmap.
Syncing $MFT.
Updating $MFTMirr.
Syncing device.
mkntfs completed successfully. Have a nice day.