hdiutil notes

Notes and other random things related to the macOS hdiutil command.

Create a loop device

Similar to losetup for Linux, this allows setting up a loop device for any file. Usually hdiutil tries to determine how to attach the file based on the file extension and contents, so we need to force it to assume a raw disk with -imagekey diskimage-class=CRawDiskImage.

hdiutil attach -imagekey diskimage-class=CRawDiskImage -nomount myfile.bin

Create a RAM drive

Create a RAM drive. The value is the amount of 512-byte sectors, so 8388608 * 512 = 4294967296 or 4.0 GiB.

The drive is not pre-formatted, therefore -nomount is required, or else hdiutil will newly-created drive will be scanned for partitions, find none because it's completely blank, and cancel attaching.

hdiutil attach -nomount ram://8388608

After attaching it can be formatted with any tool, such as diskutil, Disk Utility, or newfs_*.

Table of common sizes for quick access

Disk size	In bytes	In sectors	hdiutil command
1 GiB	1073741824	2097152	hdiutil attach -nomount ram://2097152
2 GiB	2147483648	4194304	hdiutil attach -nomount ram://4194304
3 GiB	3221225472	6291456	hdiutil attach -nomount ram://6291456
4 GiB	4294967296	8388608	hdiutil attach -nomount ram://8388608
5 GiB	5368709120	10485760	hdiutil attach -nomount ram://10485760
6 GiB	6442450944	12582912	hdiutil attach -nomount ram://12582912
7 GiB	7516192768	14680064	hdiutil attach -nomount ram://14680064
8 GiB	8589934592	16777216	hdiutil attach -nomount ram://16777216

create -srcfolder doesn't preserve creation time

Still does this as of Monterey. Creation time is reset to modified time. This isn't really an issue unless you're like me and want to preserve as much info as possible.

Currently the only way I found around this is to manually copy the files in Finder. Maybe there's another command-line tool to preserve this data. Either way though, it means the disk image needs to be manually created with the right size.

FAT32 partition created by EmuNAND9 somehow confuses hdiutil

Any time I've tried to create an image from a FAT32 partition that was created by some version of EmuNAND9 caused hdiutil to exit with hdiutil: create failed - error -4. I thought it was due to an EmuNAND or RedNAND once existing, but I found out it happens even if I try to do so with just the partition.

This error is so weird and rare that I couldn't find anyone else with it through Google. I guess I shouldn't expect hdiutil to work with weird non-standard setups though.

Works fine with an SD card formatted with GodMode9.