Very slow and unsuccessful windows 8.1 installation on latest imac 5 k retina display

If you are referring to BC Assistant taking a long time, it is related to downloading the Windows support software, which is not available by any other means.

You should not use a USB3 flash disk for installing Windows 8.1, because it has no USB3 drivers. USB3 support becomes available after Windows is fully installed and the BC drivers are fully installed, not before.

You have another option. This option is the easiest when using a non-Fusion single disk iMac 5k. Use DU to create a Free Space partition and use the DVD/USB to install using EFI boot. Your iMac is UEFI-compliant an supports this method. Once Windows is installed with generic drivers, the USB containing BC drivers (which can only be created by BCA Option 2 - Download software) can be used to install appropriate drivers.

Can you check if your flash drive is USB3 or USB2? You can check it on the OSX side by plugging it in and look at the System Report -> USB and whether it shows up in High-Speed or Super-speed bus?

Can I suggest you find a USB2 flash drive and rebuild the Installer and test? The installation may not be properly complete if a USB3 is used. You may be missing drivers.

Do you have all the BC drivers installed? Do you have any unknown devices in Device Manager?

I have exactly the same problem. I had to do a reinstall of windows and it took indeed 5 hours. It is not the USB, because everything is already on your disk during installation (with my previous install everything was really fast).

It has something to do with the disk. So windows is running but disk respons time is very high. So I'm assuming it is a driver problem. Default microsoft drivers are installed. Not sure if it suppose to be Apple versions. Bootcamp install doesn't replace the disk drivers.

Have to say that windows sees the fusion drive as two separate drivers. Looks like the HD is used (not the SSD). On the other hand if I copy a big file (4GB or so) to the disk it is really fast (normal speed).

Still looking for an solution.

BCA defaults to using the HDD as the Windows destination. On an iMac 5K (which is UEFI-compliant) W8 can also be installed via EFI boot, which is usually faster, and it can be installed on the SSD part of the Fusion drive. Apple Fusion drive only has a 120G SSD part, which can result in sizing challenges.

Please see

Can't create Windows partition on roll-your-own Fusion drive.

Possible to partition Bootcamp to SSD on custom fusion drive?

The steps are similar to a standard Fusion drive (120G SSD/1-3TB HDD).

The undocumented command used to achieve such partitioning is

diskutil cs resizestack

Usage: diskutil coreStorage resizeStack

lvUUID|MountPoint|DiskIdentifier|DeviceNode [pvUUID] size

[part1Format part1Name part1Size part2Format part2Name part2Size

part3Format part3Name part3Size ...]

Resize both a logical volume and one of its underlying physical volumes in a

single operation. A single physical volume is always chosen for the underlying

shrink or grow, even if the logical volume's logical volume group is backed by

more than one physical volume. If you do not specify a particular physical

volume, then one is chosen for you. Note that if this is a grow operation,

this verb is limited by the physical volume's partition's room to grow.

Specifying zero as the size asks for an "automatic" grow-to-fill operation.

If this is a shrink operation, you can optionally request that new partitions

be created in the newly-formed free space gap in the partition map.

Again, note that this only resizes one of the underlying physical volumes; if

you need more sophistication in managing your topology, you should use the

separate physical and logical volume resize verbs.

Example: diskutil coreStorage resizeStack

11111111-2222-3333-4444-555555555555 10g JHFS+ New 1g

The pvUUID in the command line used by BC Assistant is the one pointing to the HDD. There is no restriction on using the SSD's pvUUID in this command. You need to make sure there is enough disk space. I have a similar setup on a MacMini on a custom Fusion disk. I will post the disk layout in a few minutes.

Here is my disk layout with a Fusion drive and Bootcamp on the SSD (256G Samsung 840 Pro/1TB Hitachi HDD).

diskutil list

/dev/disk0

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *256.1 GB disk0

1: EFI EFI 209.7 MB disk0s1

2: Apple_CoreStorage 128.0 GB disk0s2

3: Apple_Boot Boot OS X 134.2 MB disk0s3

4: Microsoft Basic Data BOOTCAMP 127.7 GB disk0s4

/dev/disk1

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *1.0 TB disk1

1: EFI EFI 209.7 MB disk1s1

2: Apple_CoreStorage 999.3 GB disk1s2

3: Apple_Boot Boot OS X 650.0 MB disk1s3

/dev/disk2

#: TYPE NAME SIZE IDENTIFIER

0: Apple_HFS OSX-FusionHD *1.1 TB disk2

diskutil cs list

CoreStorage logical volume groups (1 found)

|

+-- Logical Volume Group DF140929-90E7-48E1-AC3D-C9C75BA88D1D

=========================================================

Name: OSX-FusionHD

Status: Online

Size: 1127375384576 B (1.1 TB)

Free Space: 69632 B (69.6 KB)

|

+-< Physical Volume C856A74C-EB57-4D1F-8223-0A22215218ED

| ----------------------------------------------------

| Index: 0

| Disk: disk0s2

| Status: Online

| Size: 128030257152 B (128.0 GB)

|

+-< Physical Volume 80D91A14-CCC9-47B1-B071-B9ADFF23924F

| ----------------------------------------------------

| Index: 1

| Disk: disk1s2

| Status: Online

| Size: 999345127424 B (999.3 GB)

|

+-> Logical Volume Family 019D610F-21AC-4B54-AB0A-6D5F90D520A7

----------------------------------------------------------

Encryption Status: Unlocked

Encryption Type: None

Conversion Status: NoConversion

Conversion Direction: -none-

Has Encrypted Extents: No

Fully Secure: No

Passphrase Required: No

|

+-> Logical Volume D9DC0F70-38C7-4AE6-BAC4-CFBE0F8D4F68

---------------------------------------------------

Disk: disk2

Status: Online

Size (Total): 1118844862464 B (1.1 TB)

Conversion Progress: -none-

Revertible: No

LV Name: OSX-FusionHD

Volume Name: OSX-FusionHD

Content Hint: Apple_HFS