Yet another SMART question

Hello folks,

I have two WD 500GB Caviar Black drives (WD5002AALX, both over 5 years
old) and the one with the OS (muti-booted, various flavors of Windows)
is - maybe - displaying some curious SMART numbers in that the "Seek
Error Rate" switches infrequently between 0/200/200
(threshhold/value/worse) and 0/100/253, seemingly at random times (ie,
not just when the system is rebooted). The second drive's Seek Error
Rate always shows 0/200/200. The other SMART numbers seem to be OK
(I've attached the OS drive's SMART data taken from HD Sentinel below,
just for completeness sake).

Is this normal behavior? Even though they seem to be working just fine,
given that the disks are 5+ years old maybe I should be pro-active and
replace them now. Yes, I do frequent backups but rebuilding the system
drive is still a PITA.

Any insights would be appreciated.

Thanks,
Zag


ID Attribute Thresh
Value Worse Status Data
1 Raw Read Error Rate 51
200 200 OK 000000000000
3 Spin Up Time 21
139 138 OK 000000000FB0
4 Start/Stop Count
0 98 98 OK (Always passing) 000000000993
5 Reallocated Sectors Count 140
200 200 OK 000000000000
7 Seek Error Rate 0
100 253 OK (Always passing) 000000000000
9 Power On Time Count 0
62 62 OK (Always passing) 000000006E1A
10 Spin Retry Count 0
100 100 OK (Always passing) 000000000000
11 Drive Calib Retry Count 0
100 100 OK (Always passing) 000000000000
12 Drive Power Cycle Count 0
98 98 OK (Always passing) 00000000097D
192 Power off Retract Cycle Count 0 200
200 OK (Always passing) 000000000068
193 Load/Unload Cycle Count 0
200 200 OK (Always passing) 00000000092A
194 Disk Temperature 0
113 99 OK (Always passing) 00000000001E
196 Reallocation Event Count 0
200 200 OK (Always passing) 000000000000
197 Current Pending Sector Count 0 200
200 OK (Always passing) 000000000000
198 Off-Line Uncorrectable Sector Count 0 200
200 OK (Always passing) 000000000000
199 Ultra ATA CRC Error Count 0 200
200 OK (Always passing) 000000000000
200 Write Error Rate 0
200 200 OK (Always passing) 000000000000