Yes that is low. My ata133 measures 47 Mb/s and that hasn't been defragged in ages.:-(
Yours is an ATA100 so it should be a little slower than mine.You must be using XP with an external 200gig hard drive? check;
1) device manager > IDE ATA/ATAPI controllers > primary ide channels > device 0 ( if this is your C drive) advanced settings.Is transfer mode " DMA if available" and if so what is the current transfer mode?