Is­sues with an old disk image

Mac|Life - - ASK -

How can I mount an old, but im­por­tant, disk image that’s been re­ported as hav­ing an in­valid check­sum? Read-only or com­pressed disk im­ages are checked by macOS when you try to mount them. When they’re cre­ated, a check­sum of their en­tire con­tents is saved in the file, and that’s com­pared against their cur­rent check­sum. If those dif­fer, macOS re­ports this er­ror and re­fuses to mount them.

If you can’t ac­cess another un­dam­aged copy of the disk image (from a backup, say), the only way around this is us­ing the com­mand line in Ter­mi­nal. As the image is likely to be dam­aged, this can cause prob­lems, and in the worst case could cause a ker­nel panic. So save all open work and close nonessen­tial apps. You need to be con­fi­dent that the disk image isn’t ma­li­cious in any way. Open Ter­mi­nal, type a com­mand of the fol­low­ing form and then press backspace: hdiu­til at­tach file­name.dmg -nover­ify where file­name.dmg is the name of the disk image file — you can sim­ply drag it from Fin­der onto Ter­mi­nal’s win­dow at that point in the com­mand to sub­sti­tute the com­plete path and file­name. If suc­cess­ful, you’ll see a list of the par­ti­tions that have been mounted.

When macOS dis­cov­ers a check­sum is­sue, it writes an ex­tended at­tribute of a type whose name starts with com.ap­ple.meta­data: kMDLa­bel_. Some­times, you might need to delete that us­ing the com­mand tool xattr, or the free util­ity xat­tred, although re­mov­ing it won’t en­able you to mount the disk image.

Once you’ve mounted a dam­aged disk image us­ing the hdiu­til com­mand, it may look fine, but it’s still faulty.

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