@balaton
Here is my output when using mounter to try and mount DH1: in QEMU.
DH1: is a SFS partition on the same drive as the FFS boot partition.
Hopefully this can add extra data to point people to the issue.
00001 : Mounter : o.k. = [exec] OpenDevice("sii3112ide.device",0,0x6793C610,0x00000000) = 0 [203uS]
00002 : Mounter : o.k. = LockDosList(DEVICE|READ)
00003 : Mounter : o.k. = FindDosEntry(,"DH0",DEVICE) = "DH0" [20uS]
00004 : Mounter : o.k. = FindDosEntry(,"DH1",DEVICE) = "DH1" [36uS]
00005 : Mounter : o.k. = FindDosEntry(,"DH2",DEVICE) = "DH2" [32uS]
00006 : Mounter : UnLockDosList(DEVICE|READ)
00007 : Mounter : o.k. = DismountDevice("DH1:",0x00000006,0)
00008 : Mounter : o.k. = [exec] OpenDevice("input.device",0,0x66F7D1E0,0x00000000) = 0 [123uS]
00009 : Mounter : [exec] CloseDevice("input.device") [9uS]
00010 : Mounter : o.k. = LockDosList(DEVICE|READ)
00011 : Mounter : FAIL = FindDosEntry(,"DH1",DEVICE) = "" [34uS]
00012 : Mounter : UnLockDosList(DEVICE|READ)
00013 : Mounter : o.k. = MakeDosEntry("DH1",0)
00014 : Mounter : o.k. = [exec] OpenResource("FileSystem.resource") [22uS]
00015 : Mounter : o.k. = AddDosEntry("DH1")
00016 : DH1/SmartFilesystem 1.293 : -----> PRIVATEInternalRunCommand("", ARG:"")
00017 : DH1/SmartFilesystem 1.293 : 1 = GetSegListInfo(0x00702659 [""], ) [16uS]
00018 : DH1/SmartFilesystem 1.293 : 0 = FindSegmentStackSize("") [55uS]
00019 : DH1/SmartFilesystem 1.293 : 1 = GetSegListInfo(0x00702659 [""], ) [13uS]
00020 : DH1/SmartFilesystem 1.293 : o.k. = [exec] OpenLibrary("dos.library",39) [55uS]
00021 : DH1/SmartFilesystem 1.293 : o.k. = WaitPkt(0x00000000)
00022 : DH1/SmartFilesystem 1.293 : o.k. = [exec] OpenResource("newfilesystem.resource") [16uS]
00023 : DH1/SmartFilesystem 1.293 : o.k. = [exec] OpenLibrary("expansion.library",51) [26uS]
00024 : DH1/SmartFilesystem 1.293 : [exec] CloseLibrary("expansion.library") [6uS]
00025 : DH1/SmartFilesystem 1.293 : o.k. = ReplyPkt(0x676B5054,R1=0,R2=103) [ACTION_STARTUP 0] [23uS]
00026 : DH1/SmartFilesystem 1.293 : [exec] CloseLibrary("dos.library") [6uS]
00027 : DH1/SmartFilesystem 1.293 : <----- PRIVATEInternalRunCommand("", ARG:"") = [103 (0x00000067)]
00028 : Mounter : FAIL = DeviceProc("DH1:") [20169uS]
00029 : Mounter : o.k. = [exec] OpenDevice("input.device",0,0x66F7D0C0,0x00000000) = 0 [115uS]
00030 : Mounter : [exec] CloseDevice("input.device") [14uS]
00031 : Mounter : [exec] CloseDevice("sii3112ide.device") [14uS]