Index: uspace/app/mkfat/mkfat.c
===================================================================
--- uspace/app/mkfat/mkfat.c	(revision 462b418bff979160ff8af9f9916ba81d26cc59cd)
+++ uspace/app/mkfat/mkfat.c	(revision 81e20c75433fb4887aa73c34e0befc7c58d0f4e6)
@@ -180,5 +180,5 @@
 		printf(NAME ": Block device has %" PRIuOFF64 " blocks.\n",
 		    dev_nblocks);
-		if (dev_nblocks < cfg.total_sectors)
+		if (!cfg.total_sectors || dev_nblocks < cfg.total_sectors)
 			cfg.total_sectors = dev_nblocks;
 	}