[thefoolnz]

what drive is the external?

If it is a WD green , then you may have alignment issues
all the green drives support the new advanced format which is the 4096 physical sector with a logical remapping for backward compatibility

Since the ps3 does not support ntfs , i suspect you formated it to fat32
and depending on the tool it will have left your drive misaligned, as most old tools use the old chs alignment which aligns on a cylinder boundary, and that cause the partition to start on an non even 4096 boundary,

a bit of maths:
4096 byte sector = 8 x 512 sectors
in the old system there were 63 sectors per cylinder
now notice here that 8x8 =64 <> 63 and this is where the misalignment occurs

The fix:
unfortunately the only way to correct this issue is to reformat the drive correctly
this will involve:
1) removing the backups onto another drive for temporary storage
2) formatting the drive and having the drives first partition start at 2048 LBA, you lose a megabyte but gain performance , the chice of 2048 has been selected by a large number of people for raid/windows etc so hence why you should too
3) restore your games back onto the drive

using ubuntu/linux:

1) become root, enter your password
sudo -s
2) find the disk using
fdisk -l
3) for this example i will use drive sdh, your maybe different
here we turn of cylinder use and turn of dos compatibility
fdisk -uc /dev/sdh
4) learn how many partitions exist by displaying them
p <enter>
5) create an empty dos partition table (the MBR)
o <enter>
6) next i will assume you are only creating one partition for this example primary partition 1
n <enter> <---- create new partition entry
p <enter> <---- select a primary partition
1 <enter> <---- then select partition 1

at this point my display showed:
First sector (2048-3907029167, default 2048)
simply go with the default (ah 2048 exactly what we wanted)
so press <enter> twice once for the above and once for the
default size allocated which is all of the space which was:
Last sector, +sectors or +size{K,M,G} (2048-3907029167, default 3907029167):
in my case

7) you have now just defined your partition so you need to write the changes too disk and exit fdisk
w <enter> <-- this writes the changes to disk
q <enter> <-- this quits fdisk

8) all that is left to do now is format the drive to fat32
mkfs.vfat -F 32 -n PS3Backup /dev/sdc1

otherwise no other ideas spring too mind at the moment