A .sea file is a Self-Extracting Archive. Double-clicking is all that you need to do to see the contents. Even if one will not respond correctly, dragging it onto a StuffIt 4.0.1 icon (or 5.1 or 5.5 icon or alias) should achieve the desired decompression/decoding. Disk Copy 4.2-type .img files are limited to System 6 and early System 7 downloads, but entirely unnecessary if you are getting .sea files, or even .smi (Self-Mounting Images), which also open at a double-click.
Difficulties arise in the download-and-extract process depending on the source of the downloads. Apple archived some System files differently after their general currency, and you may have acquired some of these rather than the original Disk Copy 4.2-type archives.
Jan Hedlund is one of the best advisers in the Apple Discussions about downloading System 7 through PCs. This thread deals with almost all of the ins and outs of the game, and is well worth your attention. Note also the recommendation about the superiority of a pared-down 7.5.x over 7.1 as an alternative to what you are planning.
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PS In the referenced discussion you will read about initializing transfer floppies on PCs. Given the differences between the writing characteristics of FDDs for the platforms, it is indeed better to initialize (virgin disks) and format for DOS in the Mac, write the files for transfer in the PC, and return the written disks to the Mac for use.
Difficulties arise in the download-and-extract process depending on the source of the downloads. Apple archived some System files differently after their general currency, and you may have acquired some of these rather than the original Disk Copy 4.2-type archives.
Jan Hedlund is one of the best advisers in the Apple Discussions about downloading System 7 through PCs. This thread deals with almost all of the ins and outs of the game, and is well worth your attention. Note also the recommendation about the superiority of a pared-down 7.5.x over 7.1 as an alternative to what you are planning.
de
PS In the referenced discussion you will read about initializing transfer floppies on PCs. Given the differences between the writing characteristics of FDDs for the platforms, it is indeed better to initialize (virgin disks) and format for DOS in the Mac, write the files for transfer in the PC, and return the written disks to the Mac for use.