help! my sata150 hdd slows donw at 18mb/s again!!

how long before you reformatted until you retested it to find the 20mb/s? Is the HD full? HD reads rates slow down as a drive fills up. You can see this in reviews of drives where at the beginning of the drive the rate is 60mb/s and it steadily drops to a lower amount at the end of the drive. what you are experiencing may be normal drive behavior.

TM

At this point I would suggest running some disk diagnostics on the device to check the health of the disk itself as well as the filesystem.

For the disk itself: there are tools available for download from the manufacturer of the hard drive on the company's site. These check many aspects of drive health such as bad sectors/sector remapping, head flight issues, and other so-called SMART metrics. This can give you a very good idea if there's something physically wrong with the disk that's causing this slowdown. Many tools are also available free of charge on the ultimate boot cd.

For the Filesystem: Windows provides a fine filesystem checker built-in to the OS, open My Computer->right-click on the drive->"Properties"->"Tools" tab->"Error Checking"->Check both boxes->"Ok". If this returns errors, the filessytem needs fixing (and this utility should fix the issue).

Other possible sources:
Bus-related - Is this drive the only device on the channel?
Malware/Bloatware - could be eatting resources
Device settings - Is DMA enabled? In BIOS? Windows?

Those are just the first things that come to mind.

It is true that hard drives do read more slowly at the inner tracks as opposed to the outter tracks (and they fill up from outter to inner), but not to that degree. What causes slowdowns as you fill up the disk is filesystem overhead in the form of fragmentation, which subsequently brings up a good point: he should also try defragmenting (or at least checking the disk for fragmentation)

Retest with HD Tach long test, post those results.

HD Tach ignores partitions and file systems and reads at a low level.

Running it will tell you what type of problem you are having.

thank you all !!!
i did use some hdd error check like the one from windows, pq magic 8
and maxpower from maxtor its own site. no promblems no bad sectors at all.
i'll try the HD Tach tonight

last time i deleted all partitions and redo the system, it was about a month ago, so now i have 3g free out of 7g on c: drive, and 125g free out of 180g on d: drive, both ntfs format.

this drive works on sata 1 slot, i have another WD sata drive works on sata 2, the WD one has no promblems at all, i made no changes from cables or setting from bios related to hard drives. windows only on c: drive on the maxtor disk. no partitions no system on WD disk, the WD disk works on 60mb/s all the time. i don't know what slows down the maxtor disk down. this is realy Annoying, how did it happen? now i'm even hoping the hard drive is broken so i can simply return it to maxtor  

thank you guys again!

If the disk is wrighting something the speed drops.

Do U have any progams that are runing in the background?