Hi Steve, it was my first impression, that MDR got only big blocks, but you are right, there are all sizes used. Now that I have looked more carefully, I see some inconsistency. MDR img has always the same structure, I would suspect, that block size increase with file size, but it is not true. See list of my files sorted by img size: File: SAmTopo_mdr.img, length 176291840 fat: 400h - 600h - 6200h, block 16384 File: SAmOTBR_mdr.img, length 113917952 fat: 400h - 600h - 7C00h, block 8192 File: AfricaHR_mdr.img, length 56901632 fat: 400h - 600h - 2600h, block 16384 File: SAmOTS_mdr.img, length 35307520 fat: 400h - 600h - 2C00h, block 8192 File: SAmOTN_mdr.img, length 18628608 fat: 400h - 600h - 1C00h, block 8192 File: SAmOTEC_mdr.img, length 17248256 fat: 400h - 600h - 2C00h, block 4096 File: AfricaN_mdr.img, length 13844480 fat: 400h - 600h - 1800h, block 8192 File: AfricaS_mdr.img, length 11411456 fat: 400h - 600h - 2000h, block 4096 File: AfricaW_mdr.img, length 8376320 fat: 400h - 600h - 1A00h, block 4096 File: AfricaC_mdr.img, length 7897088 fat: 400h - 600h - 1A00h, block 4096 File: AfricaE_mdr.img, length 6871040 fat: 400h - 600h - 2400h, block 2048 File: AfricaSE_mdr.img, length 4153344 fat: 400h - 600h - 1A00h, block 2048 File: AfricaSW_mdr.img, length 3403776 fat: 400h - 600h - 1000h, block 4096 File: AfricaNE_mdr.img, length 2904064 fat: 400h - 600h - E00h, block 4096 Probably something unimportant, but a bit surprising. -- Best regards, Andrzej